High frequency bridge circuits and high frequency repeaters



Feb. 21, 1939. A. ALFORD 9 HIGH FREQUENCY BRIDGE CIRCUITS AND HIGHFREQUENCY REPEATER'S Filed May 4, 1957 s Sheets-Sheen 1 FIG 2 11, 21H/l/ rmsvaavc Y SOURCE 15/ ma 'F" F/ZfQl/[IVCY r sou/m i INVENTORAMP/F514 AL FORD Feb. 21, 1939. A LFOR 2,147,809

UENCY BRIDGE CIRCUITS- AND HIGH FREQUENCY REPEATERS men FREQ 3Shets-Sheet 2 Filed May 4, 1937 4: AMPL #712 FIGAAQ 11 Z 19 we FREW/[NCYsou/ac! b \NVENTOR Y ANDREWALFORD F b. 21, 1939. A. ALFC'DRD 2,147,809

HIGH FREQUENCY BRIDGE CIRCUITS AND HIGH FREQUENCY REPEATERS FiIed May 4,1937 3 She'ets-Shee t s N B a E! ::3

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\NVENTOR Q: ANDREW 44/0/20 2 m BY Q 1: m R ATTORNEY Patented 21, 1 9 39man FREQUENCY names". onworrs AND men raEQUENoY REPEATERS Andrew Alford,New York, N. Y., assignor to Mackay Radio and Telegraph Company, NewYork, N. Y., a corporation oi Delaware Application 4, 1937, Serial No.1.405%

20 Claims.

My invention relates to high frequency units particularly for use ashigh frequency bridge circuits and in high frequency repeaters. Becauseof the extreme simplicity of the system and the ease with which veryaccurate adjustment may be made, this system is particularly useful whendealing with extremely high frequencies, for example in the neighborhoodof 2 megaeycle's;

My invention makes use of reentrant loop cir- 0 cults, and in itsbroadest form comprises a reentrant loop circuit coupled to some sourceof high frequency so that at points on said loop there may be producedvoltage nodes or current nodes which may be used as points for couplingto the desired apparatus or circuit. Such reentrant loop units may bethen used for extremely accurate comparison of impedances or forcoupling together circuits in conjugate relation in a manner similar tothat used in the conventional hybrid coil circults It is an object of myinvention to provide such a unit which may be used readily in bridgecircuits and other high frequency apparatus.

It is a further object of my invention to produce an extremely accurateimpedance bridge circuit.

It is a further object of my invention to provide a high frequencyrepeater system in which amplification and reenforcement of high,frequency waves may be accomplished readily by providing a bridgecircuit for connecting such high frequency repeater systems to a line ina very simple and easily adjusted manner.

It is a further object of my invention to provide a two-way highfrequency communication system between two points in which systeminterference 0 between the transmitter and receiver is prevented by useof reentrant bridge circuits.

Other objects and uses of my invention will be suggested by. theparticular description made in connection with the accompanyingdrawings, in which Figs. 1 and 2 illustrate a simple form of the networkparticularly for explaining the theory thereof,

Fig. 3 illustrates an embodiment of my invenment,

- Fig. 4 illustrates an embodiment of my invention which comprises ahigh frequency repeaterv Fig. 5 illustrates an embodiment of myinvention applied to a two-way communication system, .Figs. 6 and 7illustrate modification of the circult shown in Fig. 2.

tion particularly adapted for impedance measure- For a more completedevelopment of the formula applicable to bridges and reentrant loops,than is set forth in this specification, reference is made to myapplication, Serial No. 118,866, filed January 2, 1937. 5

- Referring more particularly to Fig. 1, 'i l' represents a source ofradio frequency energy connected by transmission lines it, it withreentrant loops M, It at junction points M, 23. Points 25,

217 represent points along loops 11, i9 respectively, m;

which are separated equal electrical distances from the correspondingjunction points M, 22. In the loop circuit ll, It there then exist twotraveling waves, each of which start forwardly from Junction points 2i,23 as indicated by ar- 15 rows A and B, and which after passing eachother at and 21 become back waves. The two halves of loops ii, iii areassumed to be identical in all respects, including attenuation. Sincethese traveling waves travel equal distances to juncso tion points 25,21 and are of equal amplitude at the starting points 2|, 23 they are inphase and still of equal amplitude at points 25, 21. 'Since the voltagesare of equal amplitude and are in phase at points 25, 21 they add,iorminga voltage 25 loop, but the currents are in phase but in oppositedirections and consequently cancel, producing anabsolute current node.It can thus be seen that this circuit provides a very readilyconstructed means for obtaining an absolute current node.

Fig. 2 is substantially the same as Fig. 1, with the exception that theloops are transposed at a point 3i. As a consequence of thistransposition the currents in this circuit add at points 25, 21 and thevoltages cancel, producing an absolute voltage node. Since thetransmission lines in Fig. 2 are assumed to be equal in all respects, asstated in connection with Fig. 1, the voltage across the line at 25, 21would be precisely zero if there 40 were no reflection at the point oftransposition 3|. However, since there always exists a certain amount ofreflection due to any irregularity in a transmission line, and since atransposition results in such an irregularity, there will be a smallresidual voltage at 25, 21 unless some compensating means is used tocorrect for this irregularity. "Accordingly, some such compensatingmeans is indicated at 33, which may for example consist simply of asmall capacity connected across the line. This compensating meanstogether with a slight increase in the length of the arms completelycompensate for the transposition. It should be further'noted that anyinequality in the attenuation in the two arms of the bridge circuit maybe compensated by means of suitable attenuators added to the linecircuit.

With the circuit constructed as described above and the compensatingmeans 33 properly adjusted, the voltage across points 25, 21 isextremely small compared to the voltage across points a, a, b, b, whichare spaced a distance equal to a fraction of a wavelength from points25, 21. It is this phenomenon which renders this circuit particularlyuseful as a bridge.

If a sensitive meter such as, for example, a vacuum tube voltmeter, isconnected across points 25, 21 through a suitable balanced transformer,the voltage indicated by meter 35 will be very small, substantiallyzero. However, if a load such as 31, having an impedance Z1, isconnected across the line at points a, a spaced from the points 25, 21the balance of the bridge will be upset due to reflections along thearms 21, a, 2|. Accordingly, meter 35 will show a material increase inindication. However, if a second load 39 is connected across the line atpoints 6, b spaced an equal distance on the opposite side of points 25,21 from points a, a, said second load having an impedance Z: equal inevery respect to impedance Z1, then symmetry of the circuit will berestored and the voltage node again established at points 25, 21 and themeter will again indicate a minimum reading. For this to be the case,the load Z1 must be equal in every way to the load Z1 i. e. in reactanceand resistance. Thus it will be seen that the reentrant loop circuitshown behaves in much the same manner as the ordinary impedance bridge.

I! the impedance Z1 of load 31 is high, the load should be connected ata point distant a substantial fraction of a quarter wavelength from thevoltage nodal points 25, 21. In fact, it may be noted that as thedistance between the voltage nodal points 25, 21 and points a, a isgradually increased from zero with a corresponding and opposite increaseof distance between the voltage nodal points 25, 21 and b, b, thevoltage across the line at points a, a gradually increases. Since points25, 21 represent a voltage node, the voltage across a point a smalldistance from this node will be small and consequently a very smallamount of current can be diverted into the high impedance load 31. Asthe distance is increased the voltage across the line at points a, aincreases and consequently the current diverted into the load alsoincreases with theresult that a larger meter reading is noted at 35 witha given amount of unbalance. The loop circuit I1, I9 however, is notmost sensitive with respect to unbalance whenthe distance between theload and the voltage nodal points 25, 21 is equal to a quarterwavelength. The reason for this is that when the load is located at aquarter wavelength from the voltage nodal points, such a large amount ofcurrent is diverted into the load that only a small fraction is able topenetrate as far as the points 25, 21 across which the meter 35 isconnected. For this reason there is a certain distance for every valueof impedance Z1 of the load 31 at which the loop circuit will be mostsensitive. The larger the impedance Z1 of the load 31 the greater is thedistance for maximum sensitivity. Only in cases in which the loadimpedance Z1 is of very large value, is the maximum sensitivity obtainedwhen the spacing is nearly a quarter wavelength. The actual distance atwhich the given impedance should be connected to obtain maximumsensitivity oi the loop is not very critical, however, so that exactlocation 01 the load with respect to the bridge voltage node is notnecessary.

In Fig. 3 is illustrated an application of this reentrant loop bridgecircuit for impedance measurement. Because of the extreme simplicity andaccuracy of this bridge particularly in the higher radio frequencies,this device is a useful tool for measurements of all kinds. In Fig. 3,IN represents a radio frequency source connected through transmissionlines I 08, I05 to a bridge loop arrangement I01, I09 constructed insubstantially the same manner as that described in connection with Fig.2. A suitable sensitive voltmeter I2I is coupled over balancedtransformer I23 to the voltage nodal points I I5, I I1 of the loops I01,I09. The loops I01, I09 are shown as transposed at an impedance II3 tocompensate for the reflection at the transposition point. Connectedacross loops I01, I09 at points a,,a', b, b equally spaced from pointsH5, H1 are two identical shielded transformers I25, I21 through whichthe various elements to be tested may be coupled. The loop bridge asshown may then be used for comparing any desired impedances, such asvariable or fixed condensers, tuned circuits or any other equipment. Forexample, equal lengths of identical cables I29, I3I may be connected tothe transformer and may be employed for comparing two groundedimpedances, one or both of which may be at substantially large distancesfrom the loop bridge.

Another application of a somewhat different character of the loopcircuit is shown in Fig. 4.

In this circuit the loop is used somewhat in the manner of a hybrid coilfor coupling a repeater amplifier to a transmission line. In this figurethe reentrant loop circuit comprises two loops 201, 209 constructed inthe same manner as that shown in Fig. 2. An amplifier 208 is coupledwith its input across points 2| 5, 2" which represent the voltage nodalpoints of the loop system, and with its output coupled to the points 22I, 223. It can thus be seen that in this circuit any feedback from theoutput of the amplifier to the input cannot take place as long as thebridge is maintained in balance. Across two points on the bridge, a, ais connected a transmission line 239 which is coupled across the line 2connected between stations 243 and 245. This line then presents adefinite impedance Z1, and in order that the value of this impedancewill not be momentarily disturbed this line should be maderefiectionless by the use of well known matching devices. Across theloop at points b, b spaced from points 2 I 5, 2 I 1 a distance equal toa, a is an impedance element 25I which has an'impedance Z2 equal to Z1.It can be seen that in this system energy from line 24I may betransmitted over line 239 to the input of amplifier 208, and theamplified energy transmitted back over 239 to 2 in amplified form. Dueto the loop bridge 201, 209 none of the energy from the output ofamplifier 208 can be transmitted back to the input. Accordingly, arepeater amplifier coupling is made in the line 2. Such an amplifier isparticularly useful in high frequency circuits since it is capable ofeasy constructicn and accurate adjustment. It is clear that the desiredand necessary connection of the load to the proper points on the loopand the proper impedance load may be readily chosen to adapt the systemto any circuit in which it is desired to use this type of repeaterconnection.

An adaptation of the loop repeater circuit of Fig. 4 to a radio repeateris illustrated in Fig. 4A,

III and provided with' suitable impedance matching means shouldpreferably be included in line 230, to prevent reflections therein. Thismay be a separate matching device as indicated at 238, or thetransformer may be suitably designed for this purpose. It can be readilyseen from the description given above in connection with Fig. 4,,thatsignals received upon antenna 240 may be amplified in 208 and reradiatedfrom the antenna at increased strength. The conjugate bridge circuitprevents undesirable feedback and consequent building up of oscillationsin the amplifier.

If antenna 240 is mounted at a suitably isolated location so that itsfield is substantially free from disturbing foreign obj ects, theretransmission may be made at the same frequency as the receptionwithout creating any difficulties. However, it has been found thatchanges and movements about the antenna in the vicinity thereof mayproduce an unbalance in the bridge and a consequent feedback toamplifier 200 which produces undesirable howling and distortion. Toavoid this difficulty the amplifier 200 may incorporate therein afrequency changer so that the frequency retransmitted differs from the.frequency received by a fixed amount, preferably above audibility. Theamplifier 208 then will serve to prevent this undesirable feedbackthrough the proper construction and tuning of the circuit. The frequencydifference between the signals received and transmitted need not be.large, and may be such that both signals may be ordinariily received anddetected on the same receiver.

The radio repeater circuit illustrated in Fig. 4A is particularly usefulwhere coverage of territory around a fixed reception point withbroadcast signals is desired.- The amplifier 200 may be so sensitive asto readily receive rather feeble signals and retransmit these signals ata substantially increased energy level for more ready general receptionby less sensitive receiving sets.

Another application'of the use of this loop bridge circuit as aconjugate coupling means is illustrated in Fig. 5, which shows two suchbridge circuits used in a two-way high frequency communication system.In this system a transmitter indicated at 30i is coupled over a bridgecircuit shown generally at 303, through a transmission line 305, asecond bridge circuit shown generally at 301, and a transmission line309 to a receiver 3! i. At the same station with transmitter 30! areceiver 40l is coupled to bridge circuit 303 at the voltage nodal pointof the loop bridge. Accordingly, signals transmitted from 30! cannotinterfere with the reception at 40!. An impedance M3 is bridged acrossloop circuit 303 at a point spaced from the voltage nodal point adistance equal to the distance from said nodal point to the junction ofline 305 with said bridge circuit. The impedance element M3 is madeequal in every respect to the impedance of the circuit coupled throughline 305 to the bridge. At the station atwhich receiver 3| l-is located,a transmitter MI is coupled across the voltage nodal points of loopbridge30'l, and an impedance balancing'unit 3l5 is provided across loopbridge 301, said impedance being equal to the load connected across thetransmission line 305, and spaced a distance from the voltage nodalpoints position as shown in of the loop bridge circuit equal to thespacing of line 305 therefrom. K

It can readily be seen that with this arrangement communication between30! and 3 may be carried on without any eifect upon the receiver ll orthe transmitter 4| I and likewise communication may be carried onbetween transmitter ill and receiver 40!. without affecting transmitter30l or receiver 3| i. Of course, in this system the two bridges must beproperly designed and the impedances properly matched for the desiredfrequency which it is contemplated using for communication. Likewise ifdesired, two transmitters may be provided in one circuit such as at 3!",GUI and two receivers may be provided at 3i I,

Mi to permit transmission of two messagesin one direction. However, inthis latter case. the frequencies should be slightly different toprevent interference between signals at the two receivers. Theapproximate conjugate relation between the circuits aids indiscrimination so that the receivers need not be so precisely tuned. Itthus appears that these impedance bridge units formed by the simplemethod of the use of reentrant loop structures provide an efficient,easily constructed and adjusted conjugate coupling circuit readilyapplicable to any high frequency circuits in which the conjugaterelation may be found useful.

The compensating devices used to compensate for the reflection at thetransposition point as illustrated in Figs. 2-5 may be omitted in manycases since the reflection due to transposition is normally rather smalland may be compensated only be used when extremely fine adjustment ofthe loop is desired.

Furthermore, other means of obtaining the desired space relation betweenthe two arms of the bridge may be usedin place of the trans- Figs. 2-5.For example, in place of the transposition a half wavelength of thefrequency being used may be inserted in one arm of the bridge as shownin Fig. 6. In this figure a section of transmission line indicatedshown-inserted in one side at 5, 5' and 6, 6' is of a bridge loopcircuit similar to that shown in Fig. 1. With this arrangement thevoltage nodal point will occur at a different point in the bridge asindicated at I, 1'. The operation of this bridge circuit issubstantially identical with that shown in the other figures. -However,in this case the load impedances will be inserted at points spaced fromthe points 1, I, which are not at the apex of the loop, since the addedhalf wavelength has been inserted in lieu" of the transposition. I

The loop bridge circuit may likewise be constructed with any desiredform of phase shifting unit used in place of the transposition. Fig. '7illustrates a system in which a phase shifter shown generally at 4 isinserted in one side of the loop. This phase shifter may be of any formbut should be made so as to produce a 180 phase shift at the-desiredworking frequency.

With this system circuit connections may be meter I0, and a balancingimpedance equal to the load I may be connected across the loop asindicated at l2, to produce the desired balance relation. In the circuitshown in Figs. 6 and 7, means for compensating reflection may not benecessary as the irregularities are not present in the line. However,'ifany such refiections occur in Fig. 7 due to the phase shifter 4, acompensatingmeans It may be provided to correct for such reflections.

Although in each of the embodiments illustrated the loop circuit hasbeen shown as comprising simple wire lines, it is to be understood thatany desired type of line may may be used in this system. For example,the loop may be made of concentric cable conductors, or of insulatedtwisted pairs, or any desired known type of structure.

Furthermore, the operation of the system does not require that the loopbe of some particular shape since any reflections caused by any sharpangles or irregularities in the line may be compensated for by othermeans inserted in the loop. Moreover, the loops may be constructed so asto include a part of a normal transmission line already in use, for aportion of their length, as suggested in my prior application, SerialNo. 118,866, referred to above.

What I claim is:

1. A conjugate bridge circuit comprising a reentrant loop circuit, meanscoupled to said loop at a first'point, other means coupled at a point onsaid loop substantially 180 different in distance electrically inopposite directions from the coupling point of said first named means,and substantially equal impedance means coupled to said loop on oppositesides of said second named point and at equal distances therefrom.

2. A circuit in accordance with claim 1, in which said first named meanscomprises a high frequency transmitting means, said second named meanscomprises a high frequency receiving means, and said impedance meanscomprises a transmission line circuit and a matching impedance,respectively.

3. A circuit in accordance with claim 1, in which said first named meanscomprises a source of energy, said second named means comprises asensitive measuring device, and said impedance means comprises astandard impedance and a load impedance to be measured.

4. A circuit iniaccordance with claim 1, in

which said first named means comprises the output of an amplifier. saidsecond named means comprises the input of an amplifier, and saidimpedance means'comprises a transmission line circuit and a matchingimpedance, respectively.

5. A circuit in accordance with claim 1, in which saidfirst named meanscomprises the output circuit of a radio frequency amplifier, said secondnamed means comprises the input circuit of said amplifier, and saidimpedances comprise a transmission line coupled to a radio antenna and amatching impedance, respectively.

6. A high frequency communication system comprising a high frequencytransmitter, a reentrant loop circuit coupled to said transmitter, areceiver coupled to said reentrant loop circuit at a point spaced equalelectrical distances from said transmitter, a second reentrant loopcircuit,

a second transmitter and receiver coupled to said secondloop circuit inconjugate relationship, a transmission line coupled to both saidreentrant loop circuits at points on the loops between said receiver andtransmitter, and balancing networks coupled to each reentrant loopcircuit at points located symmetrically with respect to saidtransmission line coupling point.

7. A high frequency communication system comprisinga pair of reentrantloop circuits, high frequency apparatus coupled to each of saidreentrant loop circuits at conjugate points thereon, a transmission lineconnected to said reentrant loop circuits at points between said coupledapparatus, and balancing networks coupledto said loop circuits at pointson the loop circuit symmetrically arranged with respect "to theconnection point of said transmission line.

8. A high frequency bridge circuit comprising a high frequency source, areentrant loop circuit coupled to said source, apparatus coupled to saidloop circuit at a point equi-distant electrically from said transmissionsource, and means for establishing a voltage node at the point ofconnection of said apparatus, by introducing a phase shift of 180 in onearm of said loop.

9. A high frequency circuit according to claim 8, further comprising twosubstantially equal impedance means coupled to said reentrant loopcircuits at points arranged symmetrically with respect to said apparatuscouplingpoint.

10. A repeater circuit comprising an amplifier having an input and anoutput, a reentrant loop circuit, means to couple said amplifier atsubstantially conjugate points on said loop, a transmission line fortransmitting signal energy, means for coupling said transmission line toa point on said loop intermediate said amplifier connections wherebyenergy may be conducted to and conducted from said amplifier, and meanscooperating with the opposite side of said loop from that to which thetransmission line is coupled, for maintaining the conjugate relation ofsaid circuit.

11. A repeater circuit in accordance with claim 10, in which saidtransmission line is coupled between two communicating stations.

12. A high frequency bridge circuit comprising high frequency apparatus,a reentrant loop circuit coupled to said apparatus, means coupled tosaid loop circuit at a point such that the two arms of said loop circuitbetween said point and the junction point of said loop circuit with saidapparatus present equal electrical lengths, a load coupled to saidbridge circuit at a distance from said means coupling point to providefor maximum energy transfer from said bridge, and a second means spacedat a distance equal said first named distance and on the opposite sideof said coupling point, said second means presenting an impedancesubstantially equal to that of said lead.

13. A bridge circuit comprising a source of high frequency energy, areentrant loop circuit coupled to said source so as to produce a voltagenodal point in said loop circuit, a load circuit coupled to said loop ata distance from said voltage nodal point, and means presentingsubstantially the same impedance as said load coupled'to said loop at adistance equal to that of the load from said voltage nodal point and onthe opposite side of said voltage nodal point.

14. A high frequency bridge, comprising a reentrant loop, means coupledto said loop to in troduce energy therein, said loop being soconstructed that energy introduced therein produces a definite node at apoint in said loop, means for coupling a load to said loop intermediatesaid first .named means and said nodal point, and means v arc-moo insaid loop on the other side of said nodal point to compensate for saidload and maintain the nodal point in its initial position.

1 5.A high frequency bridge as claimed in claim 14, in which the loadcoupled to the loop circuit comprises a high frequency transmissionline.

16. A high frequency repeater comprising a reentrant bridge arrangement,means for coupling a repeating means to said bridge, the input of saidrepeating means being connected to a point on the bridge which is at avoltage node with respect to the output oi said repeating means, andmeans coupled to said bridge be,- tween said voltage nodal point and theoutput .of said repeating means, for introducing energy to the input ofsaid repeating means and receiving the repeated energy from the outputof said repeating means.

17. A repeater as claimed in claim 16, further comprising means in saidloop circuit on the side of said voltage nodal point opposite saidcoupled means, for compensating for said coupled means to maintain theloop in stable condition.

18. A bridge circuit comprising energy supply means, means associatedwith said energy supply means for transmitting energy therefrom in twopaths to a common point, said two paths being so proportioned that avoltage node is produced at said common point, a load circuit coupled toone of said paths at a distance from said common point and meanspresenting sub.- stantially the same impedance as said load coupled tothe other of said paths at a corresponding distance from said commonpoint.

19. A bridge circuit in accordance with claim 18, in which saidutilization means comprises two substantially equal impedance meanscoupled on opposite sides oi said common point.

20. A bridge circuit in accordance with claim 1 18, in which said commonpoint is coupled to an amplifier, and the output 01' said amplifiercomprises the energy supply means.

ANDREW AII'ORD.

